Vacuum glazing typically comprises two confronting plane sheets of glass which are joined together and are hermetically sealed around their marginal edges. The glass sheets are separated by an evacuated space, and the separation is maintained against atmospheric pressure by an array of support pillars. The pillars typically are 0.1 to 0.2 mm high and have lateral dimensions in the order of 0.2 to 0.5 mm. Also, the pillars are distributed in the array with the spacing between neighbouring pillars being in the order of 20 to 30 mm.
The shape and dimensions of the support pillars are critical. For example, the two bearing surfaces of each pillar should be smooth and accurately parallel, whilst all pillars must have substantially the same height to within close tolerances, typically in the order of ±2 μm.
During manufacture of the vacuum glazing the pillars are placed on the upper surface of one of the glass sheets and, during this placement operation, it is essential that each pillar should stand on one of its bearing surfaces. If the pillars are incorrectly orientated they will have an inappropriate height. For example, in the case of pillars in the form of rectangular prisms, incorrectly orientated pillars will have heights that are greater than that of the surrounding pillars, and this will cause excessive stress in and localised damage to the glass sheets when the space between the sheets is evacuated.
Several methods have been described for manufacturing the support pillars.
The pillars may be made from solder glass, a low melting point glass with a coefficient of expansion close to that of the glass sheets. The solder glass is deposited as a powder, slurry or as pre-forms on one of the glass sheets. The solder glass melts during the high temperature operation that is employed to form the edge seal and fuses to the glass sheets. Thereafter, the solder glass solidifies, on cooling, to form the support pillars.
Composite pillars have also been described. Each of these pillars consists of a non-melting core which is surrounded by solder glass.
Metal pillars have also been used in vacuum glazing. Metal pillars typically are formed from sheet material using double sided photolithography and chemical or electrolytic etching. Such a process results in the formation of a pillar having an edge shape that causes it automatically to stand on one of its bearing surfaces when placed on the supporting glass sheet.
Cylindrical metal pillars have also been produced by punching. With this manufacturing method, however, it is difficult to achieve a smooth face on the surface of the pillar that is impacted during the punching operation due to dimensional characteristics of the punching tool. Some post-punching treatment is therefore normally required for such pillars to remove a burr from the impacted face. A further disadvantage of punched pillars is that they may rest on one of their sides, during positioning on the glazing, and may not stand on their bearing surfaces.
In some evacuated glazing designs it has been found that small cracks occur in the glass sheets close to the points where the pillars contact the sheets. These appear because of shear forces that are established between the pillars and the glass sheets, due to relative movement of the glass sheets. The cracks can be avoided if the ends of the pillars are coated with soft material which can deform under relative movement of the glass sheets.
It has proved difficult to employ the etching process to manufacture composite pillars that are formed with a high mechanical strength metal core and soft end caps. This is because an etchant has not been found which is compatible with the dissimilar materials of the core and the end caps.
Also, punching is not suitable for producing such a pillar because the soft metal on the impacted face of the pillar results in an even larger burr than occurs on punched pillars without soft end caps.
Ceramic materials have also been employed for use in fabricating pillars for use in vacuum glazing. Individual pillars are cut from a ceramic sheet using a diamond saw and this method would, in principle, be suitable for making composite pillars with a high strength ceramic core and soft end caps. However, a disadvantage of this method is that the pillars are cut in the form of rectangular prisms and individual pillars may therefore rest on their sides during the placement process. Furthermore, the pillars made by this method are costly because of the slow speed of the diamond sawing operation.
The present invention seeks to provide a method of producing support pillars which avoids or diminishes the above mentioned problems of the prior art methods.